1. Field of the Invention
The present invention relates to an Otto-cycle engine and, more particularly, to an engine which uses Miller cycles as well as Otto cycles and sets the expansion ratio at from 11:1 to 16:1, comparable to that of a Diesel engine, so that the compression ratio may be increased according to the operating conditions of the engine and close to the critical level at which combustion knock occurs, in order to increase the power output and improve the combustion.
2. Description of the Prior Art
Known Otto-cycle engines are designed so that the compression ratio and the expansion ratio are the same. The compression ratio is limited by combustion knock occurring in full load operation. In a non-supercharged engine, the maximum value of the compression ratio is usually about 10:1. In a supercharged engine, the compression ratio cannot be increased above about 8.5:1. Therefore, the expansion ratio assumes the value of between 8.5:1 and 10:1. High-temperature, high-pressure combustion gas produced in the cylinder does not sufficiently expand and so it is not efficiently transformed into work. The gas is then exhausted as high-temperature exhaust gas. Hence, the thermal efficiency is low.
As is well know in the art, such high-temperature exhaust gas not only reduces the thermal co-efficiency but increases the thermal stress in the cylinder head. This thermal loading may produce cracks in the head, and heat up the exhaust valve to a point where its strength is reduced. Sometimes, the exhaust breaks or is damaged.
In the case of a supercharged engine, decreases in the expansion ratio further elevate the exhaust gas temperature. In the case of turbocharging, for example, excessive thermal stress is applied to the exhaust turbine casing and other components. The permissible level may be exceeded. For these reasons, the present situation is that rich air-fuel mixture is inspirated to suppress increases in the exhaust gas temperature. Consequently, the fuel consumption is increased.
In an Otto-cycle engine taking in a mixture consisting of substantially equal ratios of fuel and air, it is a common practice to reduce the amount of the air-fuel mixture introduced into the engine with the throttle valve in order to alleviate the load. The negative pressure created by the throttle valve increases the power loss in partial load conditions. Furthermore, the density of the compressed air-fuel mixture is reduced, which in turn leads to incomplete combustion or a decrease in the rate of combustion. As a result, the indicated thermal efficiency deteriorates. Especially, when recirculation of exhaust gas and combustion of lean air-fuel mixture are effected in order to reduce the throttling loss for improving the thermal efficiency and to reduce NOx, poor combustion takes place which results in an increase in the emission of HC. Limitations may be imposed on the amount of noxious emissions. Where engine start is tried in cold areas, it is difficult to achieve low emissions because the compression ratio is not sufficiently high.
In the conventional Otto-cycle engine where the compression ratio and the expansion ratio are set to the same value, combustion knock imposes limitations on the maximum compression ratio as described above. Consequently, the expansion ratio is also restricted. This results in various problems including a decrease in the thermal efficiency under full load and decrease in the reliability of the engine due to excessively high exhaust gas temperature. In the case of turbocharging, and especially in the case of a supercharged engine where the compression ratio is set to a lower value, the exhaust gas temperature is still higher. Rich air-fuel mixture is used to lower the temperature. This not only increases the fuel consumption but also increases the thermal burden on the exhaust gas turbine of a turbocharger, and as a result the exhaust gas turbine and the casing must be made from expensive heat-resistant alloys.
Furthermore, when the engine is under partial load, decreases in the density of the compressed air-fuel mixture result in poor combustion, and a decrease in the thermal efficiency. If recirculation of exhaust gas and lean air-fuel mixture are utilized to reduce the throttling loss and NOx, then poor combustion occurs. As a result the required amount of recirculation of the exhaust gas and sufficiently lean air-fuel mixture cannot be used. It is impossible to increase the compression ratio further to elevate the compression temperature for attaining better combustion and thus enhancing the thermal efficiency.